Presently, the process used for producing butyl rubber is known, for example, from Ullmanns Encyclopedia of Industrial Chemistry, Vol. A 23, 1993, pages 288-295. The cationic copolymerization of isobutene with isoprene by the slurry process and with methylene chloride as the process solvent is carried out using aluminum trichloride as initiator with the addition of small amounts of water or hydrogen chloride at −90° C. The low polymerization temperatures are necessary in order to achieve sufficiently high molecular weights for rubber applications.
The molecular-weight-lowering (=regulating) effect of the dienic comonomers can, in principle, be compensated for by even lower reaction temperatures. In such a case, however, the side-reactions that lead to gel formation occur to a greater extent. Gel formation at reaction temperatures of about −120° C. and possible methods for the reduction thereof have been described (see: W. A. Thaler, D. J. Buckley, Sr., Meeting of the Rubber Division, ACS, Cleveland, Ohio, May 6-9, 1975, published in Rubber Chemistry & Technology 49, 960-966 (1976)). The auxiliary solvents, such as CS2, that are necessary therefor are on the one hand difficult to handle and, in addition, must be used in relatively high concentrations.
Also known is the gel-free copolymerization of isobutene with various comonomers at temperatures of about −40° C. using pre-formed vanadium tetrachloride, to form products having a sufficiently high molecular weight for rubber applications (EP-A1-818 476).
U.S. Pat. No. 2,682,531 describes zirconium tetrachloride-ether complexes and the use thereof as catalysts for the polymerization of, inter alia, isoolefins. In column 2, line 20 et seq, it is emphasized that the use of zirconium tetrachloride alone leads to unsatisfactory results. The ether that is preferably used is β,β′-dichloroethyl ether, which is carcinogenic. Diphenyl ether, which is also mentioned as an example, leads only to poorly soluble complexes that exhibit adequate activity only when metered in extremely large amounts. Diethyl ether (which is mentioned specifically as a possible ether in the patent) leads to complexes that are completely inactive.